Thick thermal barrier coating having grooves for enhanced strain tolerance

ABSTRACT

A thermal barrier coating is provided which is adapted to be formed on an article subjected to a hostile thermal environment while subjected to thermally, mechanically and/or dynamically-induced stresses, such as a component of a gas turbine engine. The thermal barrier coating is composed of a bond layer that tenaciously adheres an insulative ceramic layer to the article. The bond layer is formed of a metallic oxidation-resistant material, and has an average surface roughness R a  of at least about 7.5 micrometers, while the ceramic layer is characterized by being segmented by at least two sets of grooves. The grooves have substantially uniform widths of about 100 to about 500 micrometers, with adjacent grooves of each set being spaced about 10 to about 250 millimeters apart. The grooves promote the resistance of the thermal barrier coating to spalling, and are selectively located in the ceramic layer in order to tailor the stress relaxation capability of the thermal barrier coating to the size, geometry and service environment of the article. A preferred method for forming the grooves involves a liquid jet technique by which a portion of the ceramic layer is abraded by a high pressure liquid stream.

This invention relates to thermal barrier coatings for componentsexposed to high temperatures, such as the hostile thermal environment ofa gas turbine engine. More particularly, this invention is directed to athick thermal barrier coating characterized by grooves which are sizedand selectively spaced to enhance the strain tolerance of the thermalbarrier coating over a broad surface area, such that the thermal barriercoating resists spalling even when exposed to high stress conditionscreated by a combination of thermal, mechanical and dynamic stresses.

BACKGROUND OF THE INVENTION

Higher operating temperatures of gas turbine engines are continuouslysought in order to increase the efficiency of such engines. As operatingtemperatures increase, the high temperature durability of the componentsof the engine must correspondingly increase. A common solution is tothermally insulate certain components of a gas turbine engine in orderto minimize their service temperatures. For this purpose, thermalbarrier coatings (TBC) formed directly on the surface of a componenthave found wide use. Such coatings generally entail the deposition of ametallic bond layer onto the surface of a component, followed by aceramic layer which serves to thermally insulate the component.Preferably, the metallic bond layer is formed from anoxidation-resistant alloy in order to promote the adhesion of theceramic layer to the component. Various ceramic materials have beenemployed as the ceramic layer, particularly zirconia (ZrO₂) stabilizedby yttria (Y₂ O₃), magnesia (MgO) or other oxides. These materials canbe readily deposited by plasma spray techniques.

A primary objective of thermal barrier coating systems has been theformation of a more adherent ceramic layer which is less susceptible tospalling when subjected to thermal cycling. For this purpose, the priorart has proposed various types of coating systems, including theformation of ceramic layers having enhanced strain tolerance as a resultof the presence of porosity, microcracks and segmentation of the ceramiclayer. Microcracks generally denote random internal discontinuitieswithin the ceramic layer, while segmentation indicates the presence ofmicrocracks that extend through the thickness of the ceramic layer,thereby imparting a columnar structure to the ceramic layer.

As taught by Sumner et al. in an article entitled "Development ofImproved-Durability Plasma Sprayed Ceramic Coatings for Gas TurbineEngines", published by the AIAA/SAE/ASME 16th Joint PropulsionConference, Jun. 30 through Jul. 2, 1980, and Duvall et al. in anarticle entitled "Ceramic Thermal Barrier Coatings for Turbine EngineComponents" ASME paper 82-GT-322, the presence of microcracks orsegmentation can effectively serve as a strain relief mechanism for athermal barrier coating, as evidenced by the results of controlledthermal cyclic testing.

Similarly, U.S. Pat. No. 5,073,433 to Taylor reports that the presenceof vertical macrocracks homogeneously dispersed in a thermal barriercoating are capable of improving the thermal fatigue resistance of thecoating. While Taylor employs the term "vertical macrocracks", Taylor'sdescription depicts segmentation essentially identical to that of Sumneret al. and Duvall et al., with approximately 75 to 100 microcracks beingpresent per linear inch of coating and each microcrack having a width ofpreferably less than 1/2 mil (about 13 micrometers).

A shortcoming of the above prior art is that the advantageous resultsobtained are generally restricted to strains thermally induced betweenthe ceramic layer and the underlying substrate provided by thecomponent. More specifically, the microcracks and segmentation providestrain relaxation for only local stresses, such as those generated by amismatch in coefficients of thermal expansion of the ceramic layer andthe substrate metal. As such, the microcracks are unable to provideadequate stress relaxation if the ceramic layer is relatively thick,such as on the order of about 0.75 millimeter or more, withcorrespondingly higher residual stresses.

The microcracks are also generally inadequate to provide protection fromstrains induced by a combination of thermal, mechanical and dynamicstresses, such as extremely high temperature gradients, compressive,tensile and hoop stresses, and vibrationally-induced stresses. Whenexposed to such conditions, large areas of the thermal barrier coatingtend to spall or otherwise delaminate from the bond layer, because themicrocracks do not provide sufficiently large discontinuities to arrestthe propagation of cracks within the ceramic layer.

In addition, the methods by which the microcracks and segmentation areproduced do not permit the location, spacing and size of the microcracksto be selectively controlled. As such, the thermal fatigue properties ofcomponents equipped with these thermal barrier coatings cannot betailored to the specific geometry of a component or differences inoperating environments at various locations on a component.

While the use of thermal barriers formed of ceramic tiles tends to solvesome of the above shortcomings, their use is generally limited torelatively large articles whose applications warrant a relativelylabor-intensive assembly procedure. However, weight consequences andadhesion is often a concern with ceramic tiles. Furthermore, ceramictiles are unsuitable for protecting articles with complicatedgeometries.

Accordingly, it would be desirable to provide a thermal barrier coatingcharacterized by the ability to resist spallation when subjected tohostile environments, particularly those in which a combination ofthermal, mechanical and dynamic stresses are imposed. Preferably, such acoating would be capable of being tailored to specific applications inwhich the operating environment at various locations on an article maydiffer significantly, and in which the geometry of the article precludesthe use of tiles.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a thermal barrier coatingfor an article exposed to a hostile thermal environment whilesimultaneously subjected to thermal, mechanical and dynamic stresses.

It is a further object of this invention that such a thermal barriercoating be highly resistant to spallation, even when the coatingthickness exceeds about 2.5 millimeters.

It is another object of this invention that such a thermal barriercoating be characterized by the presence of grooves that are selectivelylocated on the article to form a grid pattern which provides strainrelaxation over a broad surface area of the article.

It is yet an another object of this invention that such grooves serve ascrack arresters that restrict spallation of the thermal barrier coatingto a limited surface region of the article.

It is still a further object of this invention to provide a method forforming such a thermal barrier coating.

The present invention generally provides a thermal barrier coating whichis adapted to be formed on an article subjected to a hostile thermalenvironment while subjected to thermally, mechanically anddynamically-induced stresses, such as components of a gas turbineengine. The thermal barrier coating is composed of a bond layeroverlaying the surface of the article, and a ceramic layer on the bondlayer. The bond layer serves to tenaciously adhere the ceramic layer tothe article, while the ceramic layer serves as a thermal insulator tothe article.

The bond layer is formed of a metallic oxidation-resistant material, andhas an average surface roughness R_(a) of at least about 7.5 micrometers(about 300 microinches). The ceramic layer is characterized by beingsegmented by grooves that are arranged to define a grid in the surfaceof the ceramic layer. The grooves have substantially uniform widths ofabout 100 to about 500 micrometers, with adjacent grooves being spacedabout 10 to about 250 millimeters apart. A preferred material for theceramic layer is an yttria-stabilized zirconia, while a preferredmaterial for the bond layer is a nickel-base alloy.

In accordance with this invention, the thermal barrier coating isadvantageously resistant to spalling due to the presence of the groovesin the ceramic layer. This capability exists even when the coatingthickness exceeds about 2.5 millimeters. The grooves are preferablyformed after the ceramic and bond layers have been deposited, and areselectively located in the ceramic layer in order to tailor the stressrelaxation capability of the thermal barrier coating to the size,geometry and service environment of the article. A preferred method forforming the grooves involves a liquid jet technique by which a portionof the ceramic layer is abraded by a high pressure liquid stream.

Notably, the grooves of this invention are significantly larger thanthat taught by the prior art, and are spaced apart a greater distancethan that possible with the prior art. As a result, the grooves enablethe thermal barrier coating to provide stress relaxation over a broadsurface area of the article on which the thermal barrier coating isformed. The grooves are also capable of a higher level of stressrelaxation, such as that imposed by a combination of thermally,mechanically and dynamically-induced stresses. The location and spacingof the grooves enable the thermal barrier coating to be tailored toapplications in which the operating environment at various locations onthe article may differ significantly, and in which the geometry of thearticle precludes the use of tiles. Finally, the grooves act as crackarresters, such that spallation of the thermal barrier coating will berestricted to a limited surface region of the article.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of this invention will become moreapparent from the following description taken in conjunction with theaccompanying drawing, in which FIG. 1 shows a representation[photomicrograph] of a cross-sectional portion of an article on which athermal barrier coating is formed in accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to metal components thatoperate within environments characterized by relatively hightemperatures, in which the components are subjected to a combination ofthermal, mechanical and dynamic stresses. Examples are the stator vanes,turbine blades and combustor liners of a gas turbine engine, which aresubjected to a combination of such stresses within the operatingenvironment of the engine. While the advantages of this invention willbe illustrated and described with reference to components of gas turbineengines, the teachings of this invention are generally applicable to anycomponent in which a thermal barrier can be used to insulate thecomponent from a hostile thermal environment.

To illustrate the invention, a cross-sectional representation of anarticle coated with a thermal barrier coating is shown in FIG. 1. As isconventional, the article may be formed from a nickel-base superalloy oranother suitable high temperature material. According to this invention,the article is thermally insulated from its hostile thermal environmentby a thermal barrier coating 10 which is uniquely adapted to beresistant to spalling and delamination as a result of high stressesbeing imposed over large areas of the article.

The coating 10 is composed of a bond layer 12 over which a ceramic layer14 is formed. The bond layer 12 is preferably formed of a metallicoxidation-resistant material, such that the bond layer 12 will beresistant to oxidation and will therefore be capable of more tenaciouslyadhering the ceramic layer 14 to the article. A preferred bond layer 12is formed by a nickel-base alloy powder, such as NiCrAlY, which has beendeposited on the surface of the article to a thickness of about 0.125 toabout 0.375 micrometers.

A preferred deposition method is an air plasma spray technique, thoughit is foreseeable that other deposition methods such as physical vapordeposition (PVD) could be used. Importantly, the alloy powder preferablyhas a coarse particle size, such as on the order of about 50 to about150 micrometers. The intent is to generate a bond layer 12 whose averagesurface roughness R_(a) is at least about 7.5 micrometers (about 300microinches), as measured in accordance with standardized measurementprocedures. According to the present invention, a surface roughness ofat least about 7.5 micrometers is necessary to achieve a suitable bondbetween the ceramic layer 14 and the bond layer 12.

The ceramic layer 14 is also preferably deposited by an air plasma spraytechnique. A preferred material for the ceramic layer 14 is anyttria-stabilized zirconia, a preferred composition being 8 weightpercent yttria and 92 weight percent zirconia, though other ceramicmaterials could foreseeably be used. The ceramic layer 14 is depositedto a thickness that is sufficient to provide the required thermalprotection for the article. In accordance with this invention,thicknesses of about 2.5 millimeters and greater are possible, contraryto prior art thermal barrier coatings whose thicknesses are limited byresidual stresses that would promote spalling and delamination of thecoating when subjected to thermal cycling.

To achieve adequate stress relaxation, the ceramic layer 14 of thisinvention is segmented by grooves 16, one of which is shown in FIG. 1.The grooves 16 are formed in the ceramic layer 14 to create a gridpattern composed of two substantially parallel sets of grooves 16, thegrooves 16 of one set being nonparallel to the grooves 16 of the otherset. The sets of grooves 16 can be, but are not required to be,perpendicular to each other. The grooves 16 are preferably formed afterthe bond and ceramic layers 12 and 14 have been deposited on thearticle. A preferred method for defining the grooves 16 uses a water jetwhich cuts the grooves 16 in the ceramic layer 14 without harming theunderlying surface of the article.

As shown in FIG. 1, the groove 16 extends down to the bond layer 12without causing any degradation of the bond coat quality, such as crackpropagation through the bond layer 12 and into the surface of thearticle. However, the depth of the grooves 16 need not extend to thebond layer 12, and can vary for the purpose of tailoring the coating 10to stress conditions in particular regions of the article. While FIG. 1shows a somewhat nonuniform taper to the walls of the groove 16,improvements in the uniformity of the groove shape and size can beaccomplished by modifying the water jet cutting technique. Overall, thegrooves 16 are very uniform as compared to the random internaldiscontinuities which characterize the microcracks and segmentations ofprior art thermal barrier coatings.

Grooves 16 formed in accordance with this invention have substantiallyuniform widths of about 100 to about 500 micrometers, and adjacentgrooves 16 within each set are spaced about 10 to about 250 millimetersapart. The preferred groove width and spacing are required to achieve asuitable stress relaxation capability over a broad surface region of thearticle, as is necessary to endure significant thermally, mechanicallyand dynamically-induced stresses, and stresses which are a combinationthereof.

Notably, the ability to selectively locate the grooves 16 on the articleenables the stress relaxation capability of the thermal barrier coating10 to be tailored to the size, geometry and service environment of thearticle. In contrast, the wholly random and uncontrollable formation ofthe microcracks of the prior art make such an advantage impossible toachieve.

A preferred method for forming the thermal barrier coating 10 of thisinvention is generally as follows. The surface of the article is firstprepared by grit blasting with alumina particles. The bond layer 12 isthen deposited in a conventional manner using an air plasma spraydeposition technique. As noted above, the material which forms the bondlayer 12 is preferably in the form of a coarse powder, and the resultingbond layer 12 preferably has an average surface roughness R_(a) is atleast about 7.5 micrometers.

The ceramic layer 14 is then deposited by air plasma spraying on thebond layer 12 with a suitable ceramic material. For some applications, aadequate thickness for the thermal barrier coating 10 is about 0.75millimeter though, according to this invention, spall-resistant coatings10 of about 2.5 millimeters and greater can be achieved to provideconsiderably improved thermal insulation. The grooves 16 are then formedby directing a high pressure liquid jet at the surface of the ceramiclayer 14. In practice, water at a pressure of about 100 to about 200 MPa(about 15,000 to about 30,000 psi) and directed through an orifice ofabout 0.125 millimeter in diameter has been found to achieve asatisfactory cutting action, though it is foreseeable that otherparameters and fluids could be used.

However, it is foreseeable that various other techniques could be usedto precisely define the grooves 16 required by this invention. Suchtechniques include mechanical cutters, such as a diamond wheel or knife,and the use of fine wires which are embedded in the ceramic layer 14during the deposition process and later removed. After the grooves 16are formed, the article preferably undergoes a vacuum heat treatment atabout 1080° C. for a duration of about four hours for the purpose ofstress relieving the thermal barrier coating 10.

The durability of thermal barrier coatings 10 formed in accordance withthis invention was investigated by coating an approximately 250 by 250millimeter (10 by 10 inch) substrate of IN-718 nickel-chromium alloy inaccordance with the above. The bond layer 12 was that of the preferredoxidation-resistant nickel-base NiCrAlY alloy, while the ceramic layer14 was the preferred 8% yttria-stabilized zirconia. The thickness of thecoating was about 0.75 to about 1 millimeter.

The grooves were then defined in the substrate using the preferred waterjet technique with process parameters of about 100 to about 170 MPapressure, and a nozzle orifice diameter of about 0.125 millimeter. Thegrooves were formed in a grid pattern composed of two sets of groovesoriented substantially perpendicular to each other, and with grooveswithin each set being spaced about 50 millimeters (2 inches) apart. Thewidths of the grooves were approximately 0.25 to about 0.5 millimeter(about 0.01 to about 0.2 inch). Three 25 millimeter (one inch) diameterspecimens, each containing an intersection between two grooves, werethen cut by water jet from the substrate and tested for thermal shockresistance.

The test entailed cyclicly exposing the specimens to a temperature ofabout 1400° C. (2550° F.), followed by rapidly cooling the specimens toabout 425° C. within a period of about twenty seconds. Each of thespecimens successfully completed 2000 cycles without any bondlinecracking being evident. Accordingly, the grooves were able to providestress relaxation for a relatively thick thermal barrier coating.Notably, microcrack techniques taught by the prior art are inadequatefor protecting such thick thermal barrier coatings (about 0.75millimeter and more) due to the correspondingly higher residual stressesthat are inherent at such thicknesses.

From the above, it can be seen that a significant advantage of thisinvention is that the thermal barrier coating 10 exhibits desirablethermal fatigue properties, as evidenced by the described thermal shockresistance test. The desirable thermal fatigue properties of the testedthermal barrier coating were attributed to the grooves which, accordingto this invention, were selectively located in the coating, weresignificantly larger than that taught by the prior art, and were spacedapart a greater distance than that possible with the prior art. As aresult, the grooves pattern of this invention provides stress relaxationover a broad surface area of the article on which the thermal barriercoating is formed, even where the coating thickness is in excess ofabout 0.75 millimeter.

In addition, the grooves 16 of this invention are also capable of ahigher level of stress relaxation, such as that imposed by a combinationof thermally, mechanically and dynamically-induced stresses. Theselective location and spacing of the grooves 16 enable the thermalbarrier coating 10 to be specifically tailored to applications in whichthe operating environment at various locations on an article may differsignificantly, and in which the geometry of the article precludes theuse of tiles. Finally, the grooves 16 of this invention are sized anddefined in a manner which enables them to serve as crack arresters, suchthat spallation of the thermal barrier coating 10 will be restricted toa limited surface region of the article.

While our invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art, such as by substituting other suitable materials, byutilizing various methods for depositing or forming the bond and ceramiclayers, or by utilizing various methods for forming the grooves.Accordingly, the scope of our invention is to be limited only by thefollowing claims.

What is claimed is:
 1. A thermal barrier coating formed on an articlefor use in an environment in which the article is subject to thermally,mechanically and dynamically-induced strains, the thermal barriercoating comprising:a metallic oxidation-resistant bond layer covering asurface of the article, the bond layer having an average surfaceroughness R_(a) of at least about 7.5 micrometers; a ceramic layeradherently formed on the bond layer, the ceramic layer being segmentedon the article by first and second sets of grooves that define a gridpattern in the ceramic layer, the grooves of the first and second setsof grooves having substantially uniform widths of about 100 to about 500micrometers, adjacent grooves within each of the first and second setsof grooves being spaced apart about 10 to about 250 millimeters.
 2. Athermal barrier coating as recited in claim 1 wherein the article is acomponent of a gas turbine engine.
 3. A thermal barrier coating asrecited in claim 1 wherein the first and second sets of grooves areoriented to be substantially perpendicular to each other.
 4. A thermalbarrier coating as recited in claim 1 wherein the bond layer is formedfrom a nickel-base metallic powder and the ceramic layer is formed froman yttria-stabilized zirconia.
 5. A thermal barrier coating as recitedin claim 1 wherein the ceramic layer has a thickness of at least about0.75 millimeter.
 6. A thermal barrier coating as recited in claim 1wherein the grooves extend to the bond layer.
 7. A thermal barriercoating as recited in claim 1 wherein the ceramic layer is a plasmasprayed layer.
 8. A coated component of a gas turbine engine, which issubjected to thermally, mechanically and dynamically-induced strains,the coated component comprising:a substrate; a metallicoxidation-resistant bond layer formed on a surface of the substrate, thebond layer having an average surface roughness R_(a) of at least about7.5 micrometers; a ceramic layer adherently formed on the bond layer,the ceramic layer being segmented on the article by first and secondsets of grooves that define a grid pattern in the ceramic layer, thegrooves of the first and second sets of grooves having substantiallyuniform widths of about 100 to about 500 micrometers, adjacent grooveswithin each of the first and second sets of grooves being spaced apartabout 10 to about 250 millimeters.
 9. A coated component as recited inclaim 8 wherein the first and second sets of grooves are oriented to besubstantially perpendicular to each other.
 10. A coated component asrecited in claim 8 wherein the bond layer is formed from a nickel-basemetallic powder and the ceramic layer is formed from anyttria-stabilized zirconia.
 11. A coated component as recited in claim 8wherein the ceramic layer has a thickness of at least about 0.75millimeter.
 12. A coated component as recited in claim 8 wherein thegrooves extend to the bond layer.
 13. A coated component as recited inclaim 8 wherein the ceramic layer is a plasma sprayed layer.